How to Build a Compound Machine to Pop a Balloon

The thumbtack that pops the balloon is a wedge-type simple machine.

All of the mechanical devices in the world can be broken down into combinations of six basic simple machines. These are the lever, inclined plane, wheel and axle, screw, wedge and pulley. Reading about these simple machines is one way to learn about them, but it can be hard for some people to visualize just how everything fits together when several of them are combined into one compound machine. A way to remedy this is to construct a model compound machine to perform a simple task like popping a balloon. This fun activity can also be turned into a science-fair project by making a poster explaining the parts of the compound machine.

Things You'll Need

• 1- by 1-foot sheet of 1- to 2-inch thick Styrofoam
• 7 pencils (4 sharpened, 3 unsharpened)
• Hot glue gun
• Clear tape
• Thumbtack
• String
• Balloon

Instructions

1. Draw a five- by four-inch rectangle on top of the Styrofoam sheet. Use a ruler to make sure that the corners are right angles. Push the four sharpened pencils point-first into the Styrofoam at the corners of the rectangle. The pencils should project vertically from the surface with their tips sunk one inch beneath it. These posts will hold up the cross pieces from which the rest of the machine will hang.

2. Use hot glue to attach unsharpened pencils to the top of the posts, with one unsharpened pencil joining together each of the pairs of posts, separated by five inches. You should now have two rectangular arches spaced four inches apart.

3. Use clear tape to attach the last unsharpened pencil to the middle of the top of one of the arches so that it hangs down and can swing forward and backward through the arch without too much resistance.

4. Hot glue the base (on the opposite side from the point) of the thumbtack to the bottom of the swinging pencil. The point of the thumbtack should sway directly from the other arch.

5. Tie a string to the swinging pencil near the base of the thumbtack. If it slides up and down the pencil, use a dab of hot glue to hold it in place. Run the string up and over the top of the other arch.

6. Tape an inflated balloon to the Styrofoam surface, a few inches in front of the point of the thumbtack. To operate the machine, pull the end of the string down, hoisting the swinging pencil up and back, then let it go. The pencil will swing down and forward, thrusting the point of the thumbtack into the balloon and popping it.

Tips & Warnings

• This machine contains four of the six simple machines. The swinging pencil is a lever and its connection to the arch from which it swings is an approximation of a wheel and axle. The thumbtack's point is a wedge that splits open the balloon, while the string going over the second arch is a simple pulley.

• Be careful not to burn yourself with the hot glue gun. The tip will grow very hot while it is in use. The glue is also hot enough to burn you if you touch it before it cools down and becomes solid.

References

• Compound Machines

Resources

• Simple Machines Learning Site